The present invention relates to the general field of undersea pipes resting on the sea bed or providing a bottom-to-surface connection for transferring hydrocarbons (or any type of fluid used in the production of such hydrocarbons), in particular for transporting or injecting gas.
The invention relates more particularly to a method serving to empty undersea pipes resting on the sea bottom and that have been subjected either to various tests prior to being commissioned, or else to preservation prior to being abandoned temporarily on the sea bed for a longer or shorter length of time, which involves filling them with fresh water or sea water.
Undersea pipes for transporting hydrocarbons, in particular gas, need to be subjected to several tests prior to commissioning them. Typically, these pre-commissioning operations combine operations of filling (with fresh water or sea water), of cleaning, of calibration, of tests under pressure, of leak testing, of emptying, of drying, of filling with nitrogen, etc. In particular, during those operations, the pipes are filled with sea water or fresh water, in particular in order to test that they are leaktight and can withstand pressure, or indeed to preserve them over the long term. Once those tests or preservation operations have been undertaken, it becomes necessary to empty the underwater pipes of the test or preservation water they contain in order to put them into production or to recover an end thereof on the sea bottom so as to continue installing them.
In general, the operation of emptying pipes while underwater is performed from a surface structure, e.g. a floating production storage and offloading (FPSO) unit, a stationary platform, or a construction ship dedicated to this operation. For these operations, the surface structure has deployed thereon high-pressure compressed air equipment (that may include equipment for generating nitrogen) that is connected to the bottom-to-surface connection of the submerged pipe that needs to be emptied (if its ends are both undersea, this activity requires mobilizing a specific flexible hose or “coiled tubing” unit constituted by a steel tube wound onto a storage drum). A string of scrapers is then pushed by the air (or the nitrogen) at high pressure as injected in this way from the surface structure into the underwater pipe, so as to expel the test (or preservation) water present inside it in controlled manner, while adjusting the speed of advance of the scrapers. The pipe it then isolated (from its environment) so as either to be raised to the surface or else to be put directly into production.
The dimensioning of the high-pressure compressed air equipment (and/or nitrogen generation equipment) used for this emptying operation depends on the depth at which the pipes are submerged, and also on their length and their diameter. Specifically, the longer and/or wider the pipe and greater the depth at which it is submerged, the greater the need for air (or nitrogen) that is highly compressed in order to empty out the test water filling such a pipe.
Thus, for applications that are becoming more and more common, in which pipes have lengths of several tens or even hundreds of kilometers and are submerged at great depths (i.e. depths of more than 400 meters (m)), the equipment used on the surface for emptying such pipes needs to be of dimensions suitable for delivering compressed air (or nitrogen) at very high pressures, possibly reaching 200 bars to 250 bars.
Equipment that is dimensioned for emptying such submerged pipes that are submerged at very great depths is therefore very voluminous and bulky; it occupies a large footprint area on the surface that receives it; at present, this area is typically of the order of 800 square meters (m2) to 1200 m2 for equipment that is to deliver compressed air at pressures that may be as great as 200 bars to 250 bars, which means that the speed of advance of the scrapers needs to be greatly restricted, entailing consequences in terms of organizing the scheduling of construction ships. Unfortunately, the space available on surface structures such as FPSO units is by its very nature greatly limited and can make it necessary to have recourse to an additional vessel suitable for receiving such compressed air equipment dedicated to emptying pipes, which ships are themselves limited in terms of available space.
When a construction ship is used, another constraint on operations of emptying pipes submerged at great depths relates to the hose or coiled tubing that connects the compressed air (or nitrogen) equipment to the submerged pipe, which hose needs specifically to be capable of withstanding the hydrostatic crushing inherent to great depths in the event of it becoming depressurized, whether or not depressurization is under control.